This invention relates to an abrasive grain, a method of producing such grain, and abrasive products incorporating such grain. More particularly, it relates to abrasive products containing fused alumina, zirconia and reduced titania.
Abrasives based on substantially pure alumina or alumina modified with 0.25% to 6% additions of minor impurity phases, or residual impurity phases derived from the original starting materials, have proved to be the most versatile and commercially important abrasive systems. They find application in the most diverse types of grinding operation involving the more common types of metal.
Although the levels of impurity are quite low their influence on the abrasives' grinding performance can be most dramatic and significant. By changing the impurities, cooling and solidification rate of the fused mass of material, a range of "alumina abrasives" has been developed over the years. The individual types have unique combinations of properties. e.g. hardness, toughness, frictional characteristics, microstructure, fracture properties, thermal behavior. etc., which have made each type ideally suited to a specific area or field of grinding applications using coated and bonded abrasive products containing them.
Until recently no commercially competitive material has been available which offered grinding properties superior to those of substantially pure aluminas, particularly in applications where the contact pressures were low to moderate.
Attempts have been made to alloy alumina with other oxides at much higher levels than had previously been used. The material showing distinct promise of success was zirconia.
Such attempts met with some success when at least 10% by weight of zirconia was fused with aluminum oxide and when the fused zirconia-alumina mixture was rapidly solidified.
U.S. Pat. No. 3,156,545, issued Nov. 10, 1964, to Kistler et al. discloses that an abrasive having a grinding removal rate comparable to the removal rate of alumina can be prepared by rapidly cooling a composition containing about 15% to 60% by volume of glass, such as silicon dioxide, to form a glassy matrix in which particles of zirconia and alumina are embedded. The resulting abrasive, however, was not substantially superior to alumina in steel removal rate.
Other alumina-zirconia alloys have, however, been disclosed in subsequent U.S. and British Patents wherein high purity alumina and zirconia are used. The products disclosed in these patents do show substantial improvements in performance, in specific areas, over alumina.
For example. U.S. Pat. No. 3,181,939, issued May 4, 1965. to Marshall et al., discloses that high strength abrasives can be obtained when from 10 to about 60% by weight of zirconia is fused with alpha alumina and the resulting fusion is rapidly cooled. The patent discloses that such abrasives are suitable for steel snagging operations, (i.e. high pressure operations) where high strength is required. The patent, however, indicates that the alpha alumina should be of high purity, usually at least 99.8% by weight aluminium oxide, and further indicates that the purity of the zirconia should be preferably at least 99%.
As disclosed in U.S. Pat. No. 3,891,408, issued Jun. 24. 1975. to Rowse et al. and U.S. Pat. No. 3,893,826, issued Jul. 6, 1975, to Quinan et al.. the best grinding and polishing abrasive characteristics are obtained when the proportions of zirconia to alumina are such that a eutectic structure is formed when the fused alumina-zirconia mixture is rapidly cooled.
U.S. Pat. No. 3,891,408 to Rowse et al. teaches the very rapid crystallization of eutectic and near eutectic molten mixes of aluminium oxide and zirconium oxide. Rowse et al. believe the optimum eutectic composition and performance in moderate pressure applications occurs at 43% by weight of zirconia, and the amount of zirconia in their abrasive grain is 35-50% by weight. The zirconia in their material is in the form of rods (or platelets) which, on the average, are less than 0.3 microns in diameter, and preferably at least 25% by weight of the zirconia is in the tetragonal crystal form. The solidified melt is made up of cells or colonies, typically 40 microns or less across their width. Groups of cells having identical orientation of microstructure form grains which typically include 2 to 100 or more cells or colonies. In crushing, the material fractures along grain and cell boundaries. The abrasive grits produced are described as having very high strength combined with highly desirable microfracture properties.
The novel and unexpected feature alleged for abrasives produced in accordance with U.S. Pat. No. 3,891,408 was that when produced at or near the eutectic composition they were outstandingly useful in "light duty applications". Abrasive grits produced in accordance with U.S. Pat. No. 3,891,408 gave improvements in excess of 100% of prior art standards when incorporated in coated abrasive products and tested in low or moderate pressure applications. When such grits were incorporated into bonded products substantial improvements were obtained in low to moderate pressure applications.
The properties of the abrasive grains produced in accordance with U.S. Pat. No. 3,891,408 are to be contrasted with the use of lower zirconia levels, e.g. 25% which leads to very tough abrasives which find utility in high pressure operations such as snagging operations.
U.S. patent application Ser. No. 868,154, filed Jan. 9, 1978, by Paul Cichy et al., and now abandoned in favor of U.S. patent application Ser. No. 31,251, filed Apr. 18, 1979, by Paul Cichy et al., and now abandoned, describes an abrasive composition comprising a rapidly solidified, co-fused mixture of from about 25 to about 50 weight percent zirconia, from about 49.2 to about 74.2 weight percent alumina, and from about 0.8 to about 2.5 weight percent silica. The disclosure of application Ser. No. 868,154, filed Jan. 9, 1978, was published in West German Published Patent Application ("Offenlegungsschrift") 2,900,007, on Jul. 12, 1979, based on a West Germany patent application filed Jan. 2, 1979, by Paul Cichy et al.